This invention relates generally to semiconductor device manufacture, and more particularly to a window in a polishing pad for use in chemical mechanical polishing (CAMP).
In the process of fabricating modem semiconductor integrated circuits (IC), it is necessary to form various material layers and structures over previously formed layers and structures. However, the underlying features can leave the top surface topography of an in-process substrate highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems in the photolithographic process. Consequently, it is desirable to effect some type of planarization of the substrate.
One method for achieving semiconductor substrate planarization or topography removal is chemical mechanical polishing (CAMP). A conventional chemical mechanical polishing (CAMP) process involves pressing a substrate against a rotating polishing pad in the presence of an abrasive slurry.
In general, there is a need to detect when the desired surface planarity or layer thickness has been reached or when an underlying layer has been exposed in order to determine whether to stop polishing. Several techniques have been developed for the in-situ detection of endpoints during the CAMP process. For example, an optical monitoring system for in-situ measuring of uniformity of a layer on a substrate during polishing of the layer has been employed. The optical monitoring system can include a light source that directs a light beam toward the substrate during polishing, a detector that measures light reflected from the substrate, and a computer that analyzes a signal from the detector and calculates whether the endpoint has been detected. In some CAMP systems, the light beam is directed toward the substrate through a window in the polishing pad. A layer of slurry is typically present between the substrate and an upper surface of the window.
In one aspect, the invention is directed to a system for polishing a substrate. The system has a polishing pad with a polishing surface, a polishing head to hold the substrate against the polishing pad during polishing, a layer of slurry on the polishing pad, a window formed in the polishing pad, and an optical monitoring system including a light source and a detector. The slurry has a first refractive index, and the window has a second refractive index close to the first refractive index of the slurry. The optical monitoring system is capable of generating a light beam and is arranged to direct the light beam during at least part of the polishing operation through the window to impinge on the substrate.
Implementations of the invention may include one or more of the following features. The second refractive index may be sufficiently close to the first refractive index that scratches on the window""s upper surface do not increase reflection or scattering of the light beam at the interface with the slurry. The second refractive index may be within about 0.07 of the first refractive index, or within about 0.045 of the first refractive index, or within about 0.03 of the first refractive index, or within about 0.01 of the first refractive index. The second refractive index may be in the range of 1.26 to 1.4. The second refractive index may be within 5.5% of the first refractive index, e.g., within 1.0% of the first refractive index. The window may be comprised of an optically clear material with negligible diffusing capabilities. The material may be silicone. The may be a fluoropolymer, such as poly(pentadecafluorooctylacrylate), poly(tetrafluoroethylene), poly(undecafluororexylacrylate), poly(nonafluropentylacrylate), poly(hepta-fluorobutylacrylate), or poly(trifluorovinylacetate). The polishing pad may have an upper portion and a lower portion, and the window may be formed in the upper portion of the polishing pad. A base window may be formed in the lower portion of the polishing pad directly beneath the window. The base window may be made of glass. The window may be tapered to have dimensions that increase away from the polishing surface.
In another aspect, the invention is directed to a polishing pad. The polishing pad has a layer with a polishing surface and a window formed in the layer that has a refractive index close to a refractive index of a polishing solution.
In another aspect, the invention is directed to a method of polishing a substrate. The method includes dispensing a polishing solution onto a polishing pad that has a polishing surface, bringing a substrate into contact with the polishing surface, creating relative motion between the substrate and the polishing pad, directing a light beam through a window in the polishing pad to impinge the substrate, and monitoring an intensity of a reflected light beam from the substrate. The polishing solution has a first refractive index, and the window has a second index of refraction that is approximately equal to the first index of refraction.
Potential advantages of the invention may include one or more of the following. The window may be formed out of an optically clear material with negligible diffusing capabilities with improved transparency. Scattering and reflecting of the light beam at the upper surface of the window due to scratches and irregularities may be reduced. Furthermore, reflection of the light beam at the interface between the window and the slurry may be reduced. Consequently the window may improve the signal-to-noise ratio in the signal from the detector. In addition, slurry leakage around the perimeter of the window is minimized by the configuration of the window in the polishing pad.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.